1.
Earthquake
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An earthquake is the shaking of the surface of the Earth, resulting from the sudden release of energy in the Earths lithosphere that creates seismic waves. Earthquakes can range in size from those that are so weak that they cannot be felt to those violent enough to people around. The seismicity or seismic activity of an area refers to the frequency, type, Earthquakes are measured using measurements from seismometers. The moment magnitude is the most common scale on which earthquakes larger than approximately 5 are reported for the entire globe and these two scales are numerically similar over their range of validity. Magnitude 3 or lower earthquakes are mostly imperceptible or weak and magnitude 7 and over potentially cause damage over larger areas. The largest earthquakes in historic times have been of magnitude slightly over 9, intensity of shaking is measured on the modified Mercalli scale. The shallower an earthquake, the damage to structures it causes. At the Earths surface, earthquakes manifest themselves by shaking and sometimes displacement of the ground, when the epicenter of a large earthquake is located offshore, the seabed may be displaced sufficiently to cause a tsunami. Earthquakes can also trigger landslides, and occasionally volcanic activity, in its most general sense, the word earthquake is used to describe any seismic event — whether natural or caused by humans — that generates seismic waves. Earthquakes are caused mostly by rupture of faults, but also by other events such as volcanic activity, landslides, mine blasts. An earthquakes point of rupture is called its focus or hypocenter. The epicenter is the point at ground level directly above the hypocenter, tectonic earthquakes occur anywhere in the earth where there is sufficient stored elastic strain energy to drive fracture propagation along a fault plane. The sides of a fault move past each other smoothly and aseismically only if there are no irregularities or asperities along the surface that increase the frictional resistance. Most fault surfaces do have such asperities and this leads to a form of stick-slip behavior, once the fault has locked, continued relative motion between the plates leads to increasing stress and therefore, stored strain energy in the volume around the fault surface. This continues until the stress has risen sufficiently to break through the asperity, suddenly allowing sliding over the portion of the fault. This energy is released as a combination of radiated elastic strain seismic waves, frictional heating of the fault surface and this process of gradual build-up of strain and stress punctuated by occasional sudden earthquake failure is referred to as the elastic-rebound theory. It is estimated that only 10 percent or less of a total energy is radiated as seismic energy. Most of the energy is used to power the earthquake fracture growth or is converted into heat generated by friction
2.
Aftershock
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An aftershock is a smaller earthquake that occurs after a previous large earthquake, in the same area of the main shock. If an aftershock is larger than the shock, the aftershock is redesignated as the main shock. Aftershocks are formed as the crust around the fault plane adjusts to the effects of the main shock. Typically, aftershocks are found up to an equal to the rupture length away from the fault plane. The pattern of aftershocks helps confirm the size of area that slipped during the main shock, Aftershocks rates and magnitudes follow several well-established empirical laws. The frequency of aftershocks decreases roughly with the reciprocal of time after the main shock and this empirical relation was first described by Fusakichi Omori in 1894 and is known as Omoris law. It is expressed as n = k where k and c are constants, a modified version of Omoris law, now commonly used, was proposed by Utsu in 1961. N = k p p is a third constant which modifies the decay rate. According to these equations, the rate of aftershocks decreases quickly with time, the rate of aftershocks is proportional to the inverse of time since the mainshock and this relationship can be used to estimate the probability of future aftershock occurrence. These patterns describe only the behavior of aftershocks, the actual times, numbers and locations of the aftershocks are stochastic. N =10 a − b M Where, N is the number of greater or equal to M M is magnitude a and b are constants In summary, there are more small aftershocks. Aftershocks are dangerous because they are unpredictable, can be of a large magnitude. An aftershock sequence is deemed to have ended when the rate of seismicity drops back to a background level, i. e. no further decay in the number of events with time can be detected. Land movement around the New Madrid is reported to be no more than 0.2 mm a year, Aftershocks on the San Andreas are now believed to top out at 10 years while earthquakes in New Madrid are considered aftershocks nearly 200 years after the 1812 New Madrid earthquake. Some scientists have tried to use foreshocks to help predict upcoming earthquakes, on the East Pacific Rise however, transform faults show quite predictable foreshock behaviour before the main seismic event. Reviews of data of past events and their foreshocks showed that they have a low number of aftershocks, seismologists use tools such as the Epidemic-Type Aftershock Sequence model to study cascading aftershocks. Following a large earthquake and aftershocks, many people have reported feeling phantom earthquakes when in fact no earthquake was taking place and this condition, known as earthquake sickness is thought to be related to motion sickness, and usually goes away as seismic activity tails off. Earthquake Aftershocks Not What They Seemed at Live Science
3.
Intraplate earthquake
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An intraplate earthquake occurs in the interior of a tectonic plate, whereas an interplate earthquake is one that occurs at a plate boundary. Interplate earthquakes, which occur at plate boundaries, are more common, nonetheless, very large intraplate earthquakes can inflict heavy damage, particularly because such areas are not accustomed to earthquakes and buildings are usually not seismically retrofitted. The surface of the Earth is made up of seven primary and eight secondary tectonic plates, the large plates move very slowly, owing to convection currents within the mantle below the crust. Because they do not all move in the direction, plates often directly collide or move laterally along each other. Relatively few earthquakes occur in environments, most occur on faults near plate margins. By definition, intraplate earthquakes do not occur near plate boundaries, compared to earthquakes near plate boundaries, intraplate earthquakes are not well understood, and the hazards associated with them may be difficult to quantify. The Charleston quake was particularly surprising because, unlike Boston and New York, in 2001, a large intraplate earthquake devastated the region of Gujarat, India. The earthquake occurred far from any plate boundaries, which meant the region above the epicenter was unprepared for earthquakes, in particular, the Kutch district suffered tremendous damage, where the death toll was over 12,000 and the total death toll was higher than 20,000. The 1888 earthquake in Río de la Plata, Brazil, was also an intraplate quake, many cities live with the seismic risk of a rare, large intraplate earthquake. The cause of earthquakes is often uncertain. In many cases, the fault is deeply buried. Under these circumstances it is difficult to calculate the exact seismic hazard for a given city, some progress is being made in understanding the fault mechanics driving these earthquakes. Scientists continue to search for the causes of earthquakes. The best success has come with detailed micro-seismic monitoring, involving dense arrays of seismometers, in this manner, very small earthquakes associated with a causative fault can be located with great accuracy, and in most cases these line up in patterns consistent with faulting. Cryoseisms can sometimes be mistaken for intraplate earthquakes, New Madrid Seismic Zone Saint Lawrence rift system Stein, S. and S. Mazzotti. Continental Intraplate Earthquakes, Science and Policy Issues, Geological Society of America, Special Paper 425
4.
Slow earthquake
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A slow earthquake is a discontinuous, earthquake-like event that releases energy over a period of hours to months, rather than the seconds to minutes characteristic of a typical earthquake. That is, they are compared to a regular earthquake. Earthquakes generate seismic waves when the rupture in the system occurs, the causes that lead to slow earthquakes have only been theoretically investigated, by the formation of longitudinal shear cracks that were analysed using mathematical models. The different distributions of initial stress, sliding frictional stress, if the initial stress minus the sliding frictional stress is low, and the specific fracture energy or the strength of the crustal material is high then slow earthquakes will occur regularly. In other words, slow earthquakes are caused by a variety of stick-slip, asperities are tiny bumps and protrusions along the faces of fractures. Faulting takes place all over Earth, faults can include convergent, divergent, and transform faults, as of 2013 some of the locations that have been recently studied for slow earthquakes include, Cascadia, California, Japan, New Zealand, Mexico, and Alaska. The locations of earthquakes can provide new insights into the behavior of normal or fast earthquakes. By observing the location of tremors associated with slow-slip and slow earthquakes, seismologists can determine the extension of the system, ETS events can last for weeks as opposed to normal earthquakes occur in a matter of seconds. Several slow-earthquake events around the world appear to have triggered major, conversely, major earthquakes trigger post-seismic creep in the deeper crust and mantle. Just like regular earthquakes, slow earthquakes can cause devastating tsunamis, the earthquake that caused this registered a magnitude of 7.8 and struck offshore the Mentawai islands in western Indonesia, causing more than 400 human casualties. Seismologists characterized it as a slow earthquake due to disproportionately large tsunami waves, rupture duration near 125 seconds, shallow near-trench slip, every five years a year-long quake of this type occurs beneath the New Zealand capital, Wellington. It was first measured in 2003, and has reappeared in 2008 and 2013 and it lasts for around a year each time, releasing as much energy as a magnitude 7 quake. C. s slow earthquakes fueled by fluid
5.
Submarine earthquake
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A submarine, undersea, or underwater earthquake is an earthquake that occurs underwater at the bottom of a body of water, especially an ocean. They are the cause of tsunamis. The magnitude can be measured scientifically by the use of the moment magnitude scale, understanding plate tectonics helps to explain the cause of submarine earthquakes. The plates converge upon one another, and one subducts below the other, or, little movements called fault creep are minor and not measurable. The plates meet with other, and if rough spots cause the movement to stop at the edges. When the rough spots can no longer hold, the release of the built-up motion releases. This area of slippage both horizontally and vertically is called the epicenter, and has the highest magnitude, and causes the greatest damage. As with an earthquake the severity of the damage is not often caused by the earthquake at the rift zone. Submarine earthquakes can also damage submarine communications cables, leading to disruption of the Internet. This is particularly common in Asia, where many submarine links cross submarine earthquake zones such as the Pacific Ring of Fire, the different ways in which tectonic plates rub against each other under the ocean or sea floor to create submarine earthquakes. The type of friction created may be due to the characteristic of the fault or the plate boundary as follows. Some of the areas of large tsunami producing submarine earthquakes are the Pacific Ring of Fire. The older, and denser plate moves below the lighter plate, the further down it moves, the hotter it becomes, until finally melting altogether at the asthenosphere and inner mantle and the crust is actually destroyed. The location where the two oceanic plates actually meet become deeper and deeper creating trenches with each successive action, there is an interplay of various densities of lithosphere rock, asthenosphere magma, cooling ocean water and plate movement for example the Pacific Ring of Fire. Therefore, the site of the sub oceanic trench will be a site of submarine earthquakes, for example the Mariana Trench, Puerto Rico Trench, and the volcanic arc along the Great Sumatran fault. A transform-fault boundary, or simply a boundary is where two plates will slide past each other, and the irregular pattern of their edges may catch on each other. The lithosphere is neither added to from the asthenosphere nor is it destroyed as in convergent plate action, rising convection currents occur where two plates are moving away from each other. In the gap, thus produced hot magma rises up, meets the sea water, cools
6.
Supershear earthquake
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A supershear earthquake is an earthquake in which the propagation of the rupture along the fault surface occurs at speeds in excess of the seismic shear wave velocity. This causes an effect analogous to a sonic boom, during seismic events along a fault surface the displacement initiates at the focus and then propagates outwards. Typically for large earthquakes the focus lies towards one end of the slip surface, theoretical studies have in the past suggested that the upper bound for propagation velocity is that of Rayleigh waves, approximately 0.92 of the shear wave velocity. Evidence of rupture propagation at velocities greater than S-wave velocities expected for the surrounding crust have been observed for large earthquakes associated with strike-slip faults. During strike-slip, the component of rupture propagation will be horizontal, in the direction of displacement. This contrasts with a dip-slip rupture where the direction of rupture propagation will be perpendicular to the displacement. 2013 Craig earthquakes, magnitude Mw 7.6 associated with strike-slip on the Queen Charlotte Fault - the first supershear event recognised on plate boundaries, slow earthquake Eric Dunhams webpage on Supershear Dynamics New Scientist article on Supershear earthquakes
7.
Fault (geology)
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In geology, a fault is a planar fracture or discontinuity in a volume of rock, across which there has been significant displacement as a result of rock-mass movement. Large faults within the Earths crust result from the action of tectonic forces. Energy release associated with movement on active faults is the cause of most earthquakes. A fault plane is the plane that represents the surface of a fault. A fault trace or fault line is the intersection of a plane with the ground surface. A fault trace is also the line commonly plotted on maps to represent a fault. Since faults do not usually consist of a single, clean fracture, the two sides of a non-vertical fault are known as the hanging wall and footwall. By definition, the wall occurs above the fault plane. This terminology comes from mining, when working a tabular ore body, because of friction and the rigidity of rocks, they cannot glide or flow past each other easily, and occasionally all movement stops. A fault in ductile rocks can also release instantaneously when the rate is too great. The energy released by instantaneous strain-release causes earthquakes, a common phenomenon along transform boundaries, slip is defined as the relative movement of geological features present on either side of a fault plane, and is a displacement vector. A faults sense of slip is defined as the motion of the rock on each side of the fault with respect to the other side. In practice, it is only possible to find the slip direction of faults. Based on direction of slip, faults can be categorized as, strike-slip. Dip-slip, offset is predominantly vertical and/or perpendicular to the fault trace, oblique-slip, combining significant strike and dip slip. The fault surface is usually vertical and the footwall moves either left or right or laterally with very little vertical motion. Strike-slip faults with left-lateral motion are known as sinistral faults. Those with right-lateral motion are known as dextral faults
8.
Epicenter
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The term was coined by the Irish seismologist Robert Mallet. The word, however, is misused to mean center. In seismology, the epicenter is the point on the Earths surface directly above the point where the fault begins to rupture, however, in larger events, the length of the fault rupture is much longer, and damage can be spread across the rupture zone. During an earthquake seismic waves propagate spherically out from the hypocenter, outside of the seismic shadow zone both types of wave can be detected but, due to their different velocities and paths through the Earth, they arrive at different times. This distance is called the distance, commonly measured in °. Once epicentral distances have been calculated from at least three seismographic measuring stations, it is a matter to find out where the epicenter was located using trilateration. Epicentral distance is used in calculating seismic magnitudes developed by Richter. Epicenter is frequently misused when not employed in the context of seismology and it is often utilized as an alternative to centre. For example, Travel is restricted in the Chinese province thought to be the epicentre of the SARS outbreak, garners Modern American Usage gives several examples of such misuse. However, Garner notes that these misusages may be metaphorical uses of the term to describe focal points of unstable and potentially destructive environments
9.
Hypocenter
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A hypocenter is the point of origin of an earthquake or a subsurface nuclear explosion. It is a synonym of the focus, confusingly, the term hypocenter is also used as a synonym for ground zero, the surface point directly beneath a nuclear airburst. An earthquakes hypocenter is the position where the energy stored in the rock is first released. This occurs directly beneath the epicenter, at a known as the focal or hypocentral depth. The focal depth can be calculated from measurements based on wave phenomena. Computing the hypocenters of foreshocks, main shock, and aftershocks of earthquakes allows the three-dimensional plotting of the fault along which movement is occurring, the wave reaches each station based upon how far away it was from the hypocenter. A number of things need to be taken into account, most importantly variations in the speed based upon the materials that it is passing through. With adjustments for velocity changes, the estimate of the hypocenter is made, then a series of linear equations is set up. The equations express the difference between the arrival times and those calculated from the initial estimated hypocenter. The system iterates until the location is pinpointed within the margin of error for the velocity computations, the dictionary definition of hypocenter at Wiktionary Media related to Hypocenters at Wikimedia Commons
10.
Shadow zone
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A seismic shadow zone is an area of the Earths surface where seismographs can only barely detect an earthquake after its seismic waves have passed through the Earth. When an earthquake occurs, seismic waves radiate out spherically from the earthquakes focus, the primary seismic waves are refracted by the liquid outer core of the Earth and are not detected between 104° and 140° from the epicenter. The secondary seismic waves pass through the liquid outer core and are not detected more than 104° from the epicenter. P waves that have converted to s-waves on leaving the outer core may be detected beyond 140 degrees. The three properties are, incompressibility, density and rigidity, p-wave velocity is equal to / p whereas S-wave velocity is equal to and so S-wave velocity is entirely dependent on the rigidity of the material it travels through. Liquids, however, have zero rigidity, hence making the S-wave velocity overall zero. P-waves, however, are only dependent on rigidity and as such still maintain some velocity when travelling through a liquid. Analysis of the seismology of various recorded earthquakes and their shadow zones, led geologist Richard Oldham to deduce in 1906 the liquid nature of the Earths outer core
11.
Seismic wave
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Many other natural and anthropogenic sources create low-amplitude waves commonly referred to as ambient vibrations. Seismic waves are studied by geophysicists called seismologists, seismic wave fields are recorded by a seismometer, hydrophone, or accelerometer. The propagation velocity of the waves depends on density and elasticity of the medium, velocity tends to increase with depth and ranges from approximately 2 to 8 km/s in the Earths crust, up to 13 km/s in the deep mantle. Earthquakes create distinct types of waves with different velocities, when reaching seismic observatories, among the many types of seismic waves, one can make a broad distinction between body waves, which travel through the Earth, and surface waves, which travel at the Earths surface. Other modes of wave propagation exist than those described in article, though of comparatively minor importance for earth-borne waves. Body waves travel through the interior of the Earth, surface waves travel across the surface. Surface waves decay more slowly with distance than body waves, which travel in three dimensions, particle motion of surface waves is larger than that of body waves, so surface waves tend to cause more damage. Body waves travel through the interior of the Earth along paths controlled by the properties in terms of density. The density and modulus, in turn, vary according to temperature, composition and this effect resembles the refraction of light waves. Two types of particle motion result in two types of waves, Primary and Secondary waves. Primary waves are waves that are longitudinal in nature. P waves are waves that travel faster than other waves through the earth to arrive at seismograph stations first. These waves can travel through any type of material, including fluids, in air, they take the form of sound waves, hence they travel at the speed of sound. Typical speeds are 330 m/s in air,1450 m/s in water, Secondary waves are shear waves that are transverse in nature. Following an earthquake event, S-waves arrive at stations after the faster-moving P-waves. S-waves can travel only through solids, as fluids do not support shear stresses, S-waves are slower than P-waves, and speeds are typically around 60% of that of P-waves in any given material. Seismic surface waves travel along the Earths surface and they can be classified as a form of mechanical surface waves. They are called surface waves, as they diminish as they get further from the surface and they travel more slowly than seismic body waves
12.
P-wave
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The continuum is made up of gases, liquids, or solids, including the Earth. P-waves can be produced by earthquakes and recorded by seismographs, in isotropic and homogeneous solids, the mode of propagation of a P-wave is always longitudinal, thus, the particles in the solid vibrate along the axis of propagation of the wave energy. Of these, density shows the least variation, so the velocity is mostly controlled by K and μ. The elastic moduli P-wave modulus, M, is defined so that M = K +4 μ /3, the precise speed varies according to the region of the Earths interior, from less than 6 km/s in the Earths crust to 13 km/s through the core. Geologist Francis Birch discovered a relationship between the velocity of P waves and the density of the material the waves are traveling in, primary and secondary waves are body waves that travel within the Earth. The motion and behavior of both P-type and S-type in the Earth are monitored to probe the structure of the Earth. Discontinuities in velocity as a function of depth are indicative of changes in phase or composition, differences in arrival times of waves originating in a seismic event like an earthquake as a result of waves taking different paths allow mapping of the Earths inner structure. P-waves travel through the layers of the Earths interior. As a result, there is a P-wave shadow zone between 103° and 142° from the focus, where the initial P-waves are not registered on seismometers. In contrast, S-waves do not travel through liquids, rather, the effectiveness of advance warning depends on accurate detection of the P-waves and rejection of ground vibrations caused by local activity as otherwise false-positive warnings will result. Earthquake Early Warning systems can be automated to allow for immediate safety actions such as issuing alerts, earthquake Early Warning Lamb waves Love wave Surface wave Photo Glossary of Earthquakes
13.
S-wave
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The S-wave moves as a shear or transverse wave, so motion is perpendicular to the direction of wave propagation. The wave moves through elastic media, and the restoring force comes from shear effects. Unlike the P-wave, the S-wave cannot travel through the outer core of the Earth. They can still appear in the inner core, when a P-wave strikes the boundary of molten. And when the S-waves hit the boundary again they will in turn create P-waves and this property allows seismologists to determine the nature of the inner core. As transverse waves, S-waves exhibit properties, such as polarization and birefringence, S-waves polarized in the horizontal plane are classified as SH-waves. If polarized in the plane, they are classified as SV-waves. When an S- or P-wave strikes an interface at an other than 90 degrees. As described above, if the interface is between a solid and liquid, S becomes P or vice versa, however, even if the interface is between two solid media, mode conversion results. If a P-wave strikes an interface, four propagation modes may result, reflected and transmitted P, similarly, if an SV-wave strikes an interface, the same four modes occur in different proportions. The exact amplitudes of all waves are described by the Zoeppritz equations. The prediction of S-waves came out of theory in the 1800s, taking the divergence of seismic wave equation in homogeneous media, instead of the curl, yields an equation describing P-wave propagation. The steady-state SH waves are defined by the Helmholtz equation u =0 where k is the wave number, earthquake Early Warning Lamb waves Longitudinal wave Love wave P-wave Rayleigh wave Seismic wave Shear wave splitting Surface wave Shearer, Peter. Aki, Keiiti, Richards, Paul G. Quantitative Seismology
14.
Seismometer
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Seismometers are instruments that measure motion of the ground, including those of seismic waves generated by earthquakes, volcanic eruptions, and other seismic sources. Records of seismic waves allow seismologists to map the interior of the Earth, the seismometer was invented by the Chinese polymath Zhang Heng in AD132 during the Han dynasty. The first Western description of a comes from the French physicist and priest Jean de Hautefeuille in 1703. The modern seismometer was developed in the 19th century by John Milne, James Alfred Ewing, seismograph is another Greek term from seismós and γράφω, gráphō, to draw. The concerning technical discipline is called seismometry, a branch of seismology, a simple seismometer that is sensitive to up-down motions of the earth can be understood by visualizing a weight hanging on a spring. The spring and weight are suspended from a frame that moves along with the earthʼs surface, as the earth moves, the relative motion between the weight and the earth provides a measure of the vertical ground motion. Any movement of the moves the frame. The mass tends not to move because of its inertia, early seismometers used optical levers or mechanical linkages to amplify the small motions involved, recording on soot-covered paper or photographic paper. In some systems, the mass is held nearly motionless relative to the frame by a negative feedback loop. The motion of the relative to the frame is measured. The voltage needed to produce this force is the output of the seismometer, in other systems the weight is allowed to move, and its motion produces a voltage in a coil attached to the mass and moving through the magnetic field of a magnet attached to the frame. This design is used in the geophones used in seismic surveys for oil. Professional seismic observatories usually have instruments measuring three axes, north-south, east-west, and the vertical, if only one axis is measured, this is usually the vertical because it is less noisy and gives better records of some seismic waves. The foundation of a station is critical. A professional station is mounted on bedrock. The best mountings may be in deep boreholes, which avoid thermal effects, ground noise and tilting from weather, other instruments are often mounted in insulated enclosures on small buried piers of unreinforced concrete. Reinforcing rods and aggregates would distort the pier as the temperature changes, a site is always surveyed for ground noise with a temporary installation before pouring the pier and laying conduit. Originally, European seismographs were placed in an area after a destructive earthquake
15.
Earthquake prediction
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Prediction can be further distinguished from earthquake warning systems, which upon detection of an earthquake, provide a real-time warning of seconds to neighboring regions that might be affected. In the 1970s, scientists were optimistic that a method for predicting earthquakes would soon be found. Demonstrably successful predictions of earthquakes have not occurred and the few claims of success are controversial. For example, the most famous claim of a prediction is that alleged for the 1975 Haicheng earthquake. A later study said there was no valid short-term prediction. Extensive searches have reported many possible earthquake precursors, but, so far, predictions are deemed significant if they can be shown to be successful beyond random chance. Therefore, methods of statistical hypothesis testing are used to determine the probability that a such as is predicted would happen anyway. The predictions are then evaluated by testing whether they correlate with actual earthquakes better than the null hypothesis, in many instances, however, the statistical nature of earthquake occurrence is not simply homogeneous. Clustering occurs in space and time. In southern California about 6% of M≥3.0 earthquakes are followed by an earthquake of magnitude within 5 days and 10 km. In central Italy 9. 5% of M≥3.0 earthquakes are followed by an event within 30 km and 48 hours. It has been shown that a method based solely on clustering can successfully predict about 5% of earthquakes. E. g. it has reported that members of the Chinese Academy of Sciences were purged for having ignored scientific predictions of the disastrous Tangshan earthquake of summer 1976. But warning of an earthquake that does not occur also incurs a cost, not only the cost of the emergency measures themselves, false alarms, including alarms that are canceled, also undermine the credibility, and thereby the effectiveness, of future warnings. This was prompted by more than 30 unofficial earthquake warnings, in the past three years, none of which has been accurate. The acceptable trade-off between missed quakes and false alarms depends on the valuation of these outcomes. The rate of occurrence of both must be considered when evaluating any prediction method and he found that earthquakes kill less than ten people per year in Greece, and that most of those fatalities occurred in large buildings with identifiable structural issues. Therefore, Stiros stated that it would be much more cost-effective to focus efforts on identifying and upgrading unsafe buildings, Earthquake prediction may be intrinsically impossible
16.
Earthquake forecasting
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Both forecasting and prediction of earthquakes are distinguished from earthquake warning systems, which upon detection of an earthquake, provide a real-time warning to regions that might be affected. In the 1970s, scientists were optimistic that a method for predicting earthquakes would soon be found. Demonstrably successful predictions of earthquakes have not occurred and the few claims of success are controversial. Consequently, many scientific and government resources have been used for probabilistic seismic hazard estimates rather than prediction of individual earthquakes, such estimates are used to establish building codes, insurance rate structures, awareness and preparedness programs, and public policy related to seismic events. In addition to regional earthquake forecasts, such seismic hazard calculations can take such as local geological conditions into account. Anticipated ground motion can then be used to building design criteria. Methods for earthquake forecasting generally look for trends or patterns that lead to an earthquake, as these trends may be complex and involve many variables, advanced statistical techniques are often needed to understand them, therefore these are sometimes called statistical methods. These approaches tend to have long time periods, making them useful for earthquake forecasting. Even the stiffest of rock is not perfectly rigid, given a large force the earths crust will bend or deform. According to the elastic theory of Reid, eventually the deformation becomes great enough that something breaks. Slippage along the break allows the rock on each side to rebound to a deformed state. In the process energy is released in various forms, including seismic waves, the cycle of tectonic force being accumulated in elastic deformation and released in a sudden rebound is then repeated. The most studied earthquake faults appear to have distinct segments, the characteristic earthquake model postulates that earthquakes are generally constrained within these segments. As the lengths and other properties of the segments are fixed and these include the maximum magnitude, and the amount of accumulated strain needed to rupture the fault segment. Return periods are used for forecasting other rare events, such as cyclones and floods. Instrumentation was put in place in hopes of detecting precursors of the anticipated earthquake, however, forecasted earthquake did not occur until 2004. The failure of the Parkfield prediction experiment has raised doubt as to the validity of the characteristic earthquake model itself, at the contact where two tectonic plates slip past each other every section must eventually slip, as none get left behind. But they do not all slip at the time, different sections will be at different stages in the cycle of strain accumulation
17.
Shear wave splitting
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Shear wave splitting, also called seismic birefringence, is the phenomenon that occurs when a polarized shear wave enters an anisotropic medium. The incident shear wave splits into two polarized shear waves, Shear wave splitting is typically used as a tool for testing the anisotropy of an area of interest. These measurements reflect the degree of anisotropy and lead to an understanding of the area’s crack density. We can think of the anisotropy of an area as a black box. An incident shear wave may enter an anisotropic medium from a media by encountering a change in the preferred orientation or character of the medium. When a polarized shear wave enters a new, anisotropic medium, one of these shear waves will be faster than the other and oriented parallel to the cracks or crystals in the medium. The second wave will be slower than the first and sometimes orthogonal to both the first shear wave and the cracks or crystals in the media, the time delays observed between the slow and fast shear waves give information about the density of cracks in the medium. The orientation of the fast shear wave records the direction of the cracks in the medium, when plotted using polarization diagrams, the arrival of split shear waves can be identified by the abrupt changes in direction of the particle motion. Hess made the first measurements of P-wave azimuthal velocity variations in oceanic basins and this area was chosen for this study because oceanic basins are made of large, relatively uniform homogeneous rocks. This concept was tested using seismic refraction profiles from the Mendocino Fracture Zone, hess found that the slow shear waves propagated perpendicular to the plane of slip and the higher velocity component was parallel to it. He inferred that the structure of oceanic basins could be recorded quickly, ando focused on identifying shear-wave anisotropy in the upper mantle. This study focused on shear wave splitting recorded near the Chubu Volcanic Area in Japan, using newly implemented telemetric seismographic stations, they were able to record both P-wave and S-wave arrivals from earthquakes up to 260 km beneath the volcanic area. The depths of these earthquakes make this area ideal for studying the structure of the upper mantle and they noted the arrivals of two distinct shear waves with different polarizations approximately 0.7 seconds apart. It was concluded that the splitting was not caused by the earthquake source, data from other nearby stations were used to constrain the source of the seismic anisotropy. He found the anisotropy to be consistent with the area directly below the area and was hypothesized to occur due to oriented crystals in a deep rooted magma chamber. If the magma chamber contained elliptical inclusions oriented approximately N-S, then the maximum velocity direction would also be N-S, Crampin proposed the theory of earthquake prediction using shear wave splitting measurements. This theory is based on the fact that microcracks between the grains or crystals in rocks will open wider than normal at high stress levels, after the stress subsides, the microcracks will return to their original positions. This phenomenon of cracks opening and closing in response to changing stress conditions is called dilatancy, once the signatures for an area are recognized, they may then be applied to predict nearby earthquakes with the same signatures
18.
Earthquake engineering
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Earthquake engineering is an interdisciplinary branch of engineering that designs and analyzes structures, such as buildings and bridges, with earthquakes in mind. Its overall goal is to such structures more resistant to earthquakes. An earthquake engineer aims to construct structures that will not be damaged in minor shaking, the main objectives of earthquake engineering are, Foresee the potential consequences of strong earthquakes on urban areas and civil infrastructure. Design, construct and maintain structures to perform at earthquake exposure up to the expectations, a properly engineered structure does not necessarily have to be extremely strong or expensive. It has to be designed to withstand the seismic effects while sustaining an acceptable level of damage. Seismic loading means application of an earthquake-generated excitation on a structure and it happens at contact surfaces of a structure either with the ground, with adjacent structures, or with gravity waves from tsunami. The loading that is expected at a location on the Earths surface is estimated by engineering seismology. It is related to the hazard of the location. Earthquake or seismic performance defines an ability to sustain its main functions, such as its safety and serviceability, at. A structure is considered safe if it does not endanger the lives. A structure may be considered if it is able to fulfill its operational functions for which it was designed. On the other hand, it should remain operational for more frequent, engineers need to know the quantified level of the actual or anticipated seismic performance associated with the direct damage to an individual building subject to a specified ground shaking. Such an assessment may be performed either experimentally or analytically, Experimental evaluations are expensive tests that are typically done by placing a model of the structure on a shake-table that simulates the earth shaking and observing its behavior. Such kinds of experiments were first performed more than a century ago, only recently has it become possible to perform 1,1 scale testing on full structures. Due to the nature of such tests, they tend to be used mainly for understanding the seismic behavior of structures, validating models. Thus, once validated, computational models and numerical procedures tend to carry the major burden for the seismic performance assessment of structures. The technique as a concept is a relatively recent development. In general, seismic structural analysis is based on the methods of structural dynamics, for decades, the most prominent instrument of seismic analysis has been the earthquake response spectrum method which also contributed to the proposed building codes concept of today
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Seismite
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Seismites are sedimentary beds and structures deformed by seismic shaking. The German paleontologist Adolf Seilacher first used the term in 1969 to describe earthquake-deformed layers, today, the term is applied to both sedimentary layers and soft sediment deformation structures formed by shaking. This subtle change in usage accommodates structures that may not remain within a layer, caution is urged when applying the term to features observed in the field, as similar-looking features may be products of either seismic or non-seismic perturbation. Several informal classification systems exist to help distinguish seismites from other soft-sediment deformation features, though a formal. Geologists use seismites, in combination with other evidence, to understand the earthquake history of an area. If age and distribution of seismically-generated features can be determined, then recurrence interval and seismic hazard risk can be assessed
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Ancient Greek
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Ancient Greek includes the forms of Greek used in ancient Greece and the ancient world from around the 9th century BC to the 6th century AD. It is often divided into the Archaic period, Classical period. It is antedated in the second millennium BC by Mycenaean Greek, the language of the Hellenistic phase is known as Koine. Koine is regarded as a historical stage of its own, although in its earliest form it closely resembled Attic Greek. Prior to the Koine period, Greek of the classic and earlier periods included several regional dialects, Ancient Greek was the language of Homer and of fifth-century Athenian historians, playwrights, and philosophers. It has contributed many words to English vocabulary and has been a subject of study in educational institutions of the Western world since the Renaissance. This article primarily contains information about the Epic and Classical phases of the language, Ancient Greek was a pluricentric language, divided into many dialects. The main dialect groups are Attic and Ionic, Aeolic, Arcadocypriot, some dialects are found in standardized literary forms used in literature, while others are attested only in inscriptions. There are also several historical forms, homeric Greek is a literary form of Archaic Greek used in the epic poems, the Iliad and Odyssey, and in later poems by other authors. Homeric Greek had significant differences in grammar and pronunciation from Classical Attic, the origins, early form and development of the Hellenic language family are not well understood because of a lack of contemporaneous evidence. Several theories exist about what Hellenic dialect groups may have existed between the divergence of early Greek-like speech from the common Proto-Indo-European language and the Classical period and they have the same general outline, but differ in some of the detail. The invasion would not be Dorian unless the invaders had some relationship to the historical Dorians. The invasion is known to have displaced population to the later Attic-Ionic regions, the Greeks of this period believed there were three major divisions of all Greek people—Dorians, Aeolians, and Ionians, each with their own defining and distinctive dialects. Often non-west is called East Greek, Arcadocypriot apparently descended more closely from the Mycenaean Greek of the Bronze Age. Boeotian had come under a strong Northwest Greek influence, and can in some respects be considered a transitional dialect, thessalian likewise had come under Northwest Greek influence, though to a lesser degree. Most of the dialect sub-groups listed above had further subdivisions, generally equivalent to a city-state and its surrounding territory, Doric notably had several intermediate divisions as well, into Island Doric, Southern Peloponnesus Doric, and Northern Peloponnesus Doric. The Lesbian dialect was Aeolic Greek and this dialect slowly replaced most of the older dialects, although Doric dialect has survived in the Tsakonian language, which is spoken in the region of modern Sparta. Doric has also passed down its aorist terminations into most verbs of Demotic Greek, by about the 6th century AD, the Koine had slowly metamorphosized into Medieval Greek
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Linear elasticity
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Linear elasticity is the mathematical study of how solid objects deform and become internally stressed due to prescribed loading conditions. Linear elasticity models materials as continua, linear elasticity is a simplification of the more general nonlinear theory of elasticity and is a branch of continuum mechanics. The fundamental linearizing assumptions of linear elasticity are, infinitesimal strains or small deformations, in addition linear elasticity is valid only for stress states that do not produce yielding. These assumptions are reasonable for many engineering materials and engineering design scenarios, linear elasticity is therefore used extensively in structural analysis and engineering design, often with the aid of finite element analysis. The system of equations is completed by a set of linear algebraic constitutive relations. For elastic materials, Hookes law represents the behavior and relates the unknown stresses. Note, the Einstein summation convention of summing on repeated indices is used below and these are 3 independent equations with 6 independent unknowns. Strain-displacement equations, ε i j =12 where ε i j = ε j i is the strain and these are 6 independent equations relating strains and displacements with 9 independent unknowns. The equation for Hookes law is, σ i j = C i j k l ε k l where C i j k l is the stiffness tensor and these are 6 independent equations relating stresses and strains. An elastostatic boundary value problem for a media is a system of 15 independent equations. Specifying the boundary conditions, the value problem is completely defined. To solve the two approaches can be taken according to boundary conditions of the boundary value problem, a displacement formulation. In isotropic media, the stiffness tensor gives the relationship between the stresses and the strains, for an isotropic medium, the stiffness tensor has no preferred direction, an applied force will give the same displacements no matter the direction in which the force is applied. If the medium is homogeneous, then the elastic moduli will be independent of the position in the medium, the constitutive equation may now be written as, σ i j = K δ i j ε k k +2 μ. This expression separates the stress into a part on the left which may be associated with a scalar pressure. A simpler expression is, σ i j = λ δ i j ε k k +2 μ ε i j where λ is Lamés first parameter. More simply, ε i j =12 μ σ i j − ν E δ i j σ k k =1 E where ν is Poissons ratio and E is Youngs modulus. Elastostatics is the study of linear elasticity under the conditions of equilibrium, in all forces on the elastic body sum to zero
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Earth
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Earth, otherwise known as the World, or the Globe, is the third planet from the Sun and the only object in the Universe known to harbor life. It is the densest planet in the Solar System and the largest of the four terrestrial planets, according to radiometric dating and other sources of evidence, Earth formed about 4.54 billion years ago. Earths gravity interacts with objects in space, especially the Sun. During one orbit around the Sun, Earth rotates about its axis over 365 times, thus, Earths axis of rotation is tilted, producing seasonal variations on the planets surface. The gravitational interaction between the Earth and Moon causes ocean tides, stabilizes the Earths orientation on its axis, Earths lithosphere is divided into several rigid tectonic plates that migrate across the surface over periods of many millions of years. About 71% of Earths surface is covered with water, mostly by its oceans, the remaining 29% is land consisting of continents and islands that together have many lakes, rivers and other sources of water that contribute to the hydrosphere. The majority of Earths polar regions are covered in ice, including the Antarctic ice sheet, Earths interior remains active with a solid iron inner core, a liquid outer core that generates the Earths magnetic field, and a convecting mantle that drives plate tectonics. Within the first billion years of Earths history, life appeared in the oceans and began to affect the Earths atmosphere and surface, some geological evidence indicates that life may have arisen as much as 4.1 billion years ago. Since then, the combination of Earths distance from the Sun, physical properties, in the history of the Earth, biodiversity has gone through long periods of expansion, occasionally punctuated by mass extinction events. Over 99% of all species that lived on Earth are extinct. Estimates of the number of species on Earth today vary widely, over 7.4 billion humans live on Earth and depend on its biosphere and minerals for their survival. Humans have developed diverse societies and cultures, politically, the world has about 200 sovereign states, the modern English word Earth developed from a wide variety of Middle English forms, which derived from an Old English noun most often spelled eorðe. It has cognates in every Germanic language, and their proto-Germanic root has been reconstructed as *erþō, originally, earth was written in lowercase, and from early Middle English, its definite sense as the globe was expressed as the earth. By early Modern English, many nouns were capitalized, and the became the Earth. More recently, the name is simply given as Earth. House styles now vary, Oxford spelling recognizes the lowercase form as the most common, another convention capitalizes Earth when appearing as a name but writes it in lowercase when preceded by the. It almost always appears in lowercase in colloquial expressions such as what on earth are you doing, the oldest material found in the Solar System is dated to 4. 5672±0.0006 billion years ago. By 4. 54±0.04 Gya the primordial Earth had formed, the formation and evolution of Solar System bodies occurred along with the Sun
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Earthquake environmental effects
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Both surface deformation and faulting and shaking-related geological effects not only leave permanent imprints in the environment, but also dramatically affect human structures. Moreover, underwater fault ruptures and seismically-triggered landslides can generate destructive tsunami waves, eEEs represent a significant source of hazard, especially during large earthquakes. This was observed for example during more or less catastrophic seismic events recently occurred in different parts of the world. Secondary effects, mostly this is the intensity of the ground shaking, the importance of a tool to measure earthquake Intensity was already outlined early in the 1990s. In 2007 the Environmental Seismic Intensity scale was released, a new seismic intensity scale based only on the characteristics, size, earthquake light Richter magnitude scale EEE Catalogue INQUA TERPRO Commission on Paleoseismology and Active Tectonics
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Tsunami
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A tsunami or tidal wave, also known as a seismic sea wave, is a series of waves in a water body caused by the displacement of a large volume of water, generally in an ocean or a large lake. Unlike normal ocean waves which are generated by wind, or tides which are generated by the pull of the Moon and Sun. Tsunami waves do not resemble normal undersea currents or sea waves, Tsunamis generally consist of a series of waves with periods ranging from minutes to hours, arriving in a so-called internal wave train. Wave heights of tens of metres can be generated by large events, numerous terms are used in the English language to describe waves created in a body of water by the displacement of water, however, none of the terms in frequent use are entirely accurate. The term tsunami, meaning harbour wave in literal translation, comes from the Japanese 津波, while not entirely accurate, as tsunami are not restricted to harbours, tsunami is currently the term most widely accepted by geologists and oceanographers. Tsunami are sometimes referred to as tidal waves and this once-popular term derives from the most common appearance of tsunami, which is that of an extraordinarily high tidal bore. Although the meanings of tidal include resembling or having the form or character of the tides, use of the tidal wave is discouraged by geologists. The term seismic sea wave also is used to refer to the phenomenon, prior to the rise of the use of the term tsunami in English-speaking countries, scientists generally encouraged the use of the term seismic sea wave rather than tidal wave. The Sumatran region is not unused to tsunamis either, with earthquakes of varying magnitudes regularly occurring off the coast of the island, Tsunamis are an often underestimated hazard in the Mediterranean Sea and parts of Europe. The tsunami claimed more than 123,000 lives in Sicily, the Storegga Slide in the Norwegian sea and some examples of tsunamis affecting the British Isles refer to landslide and meteotsunamis predominantly and less to earthquake-induced waves. The cause, in my opinion, of this phenomenon must be sought in the earthquake, at the point where its shock has been the most violent the sea is driven back, and suddenly recoiling with redoubled force, causes the inundation. Without an earthquake I do not see how such an accident could happen, the principal generation mechanism of a tsunami is the displacement of a substantial volume of water or perturbation of the sea. This displacement of water is attributed to either earthquakes, landslides, volcanic eruptions, glacier calvings or more rarely by meteorites. The waves formed in this way are then sustained by gravity, tides do not play any part in the generation of tsunamis. Tsunami can be generated when the sea floor abruptly deforms and vertically displaces the overlying water and they grow in height when they reach shallower water, in a wave shoaling process described below. A tsunami can occur in any state and even at low tide can still inundate coastal areas. On April 1,1946, the 8.6 Mw Aleutian Islands earthquake occurred with a maximum Mercalli intensity of VI and it generated a tsunami which inundated Hilo on the island of Hawaii with a 14-metre high surge. Between 165 and 173 were killed, the area where the earthquake occurred is where the Pacific Ocean floor is subducting under Alaska
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Seismic source
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This article is about artificial seismic sources. For natural seismic sources, see earthquake, volcano, and related articles, a seismic source is a device that generates controlled seismic energy used to perform both reflection and refraction seismic surveys. A seismic source can be simple, such as dynamite, or it can use more sophisticated technology, Seismic sources can provide single pulses or continuous sweeps of energy, generating seismic waves, which travel through a medium such as water or layers of rocks. Some of the waves then reflect and refract and are recorded by receivers, the returning signals from the sources are detected by seismic sensors in known locations relative to the position of the source. The recorded signals are then subjected to specialist processing and interpretation to yield information about the subsurface. The most basic seismic source is a hammer, either striking the ground directly, or more commonly striking a metal plate on the ground, known as hammer. Useful for seismic refraction surveys down to about 20 m below surface, explosives, such as dynamite, can be used as crude but effective sources of seismic energy. For instance, hexanitrostilbene was the main fill in the thumper mortar round canisters used as part of the Apollo Lunar Active Seismic Experiments. Generally, the charges are placed between 6 and 76 metres below ground, in a hole that is drilled with dedicated drilling equipment for this purpose. This type of drilling is often referred to as Shot Hole Drilling. A common drill rig used for Shot Hole Drilling is the ARDCO C-1000 drill mounted on an ARDCO K 4X4 buggy and these drill rigs often use water or air to assist the drilling. An air gun is used for reflection and refraction surveys. It consists of one or more pneumatic chambers that are pressurized with compressed air at pressures from 14 to 21 MPa, air guns are submerged below the water surface, and towed behind a ship. Air guns are made from the highest grades of corrosion resistant stainless steel, large chambers tend to give low frequency signals, and the small chambers give higher frequency signals. A plasma sound source, otherwise called a spark gap sound source, for each firing, electric charge is stored in a large high-voltage bank of capacitors, and then released in an arc across electrodes in the water. The underwater spark discharge produces a plasma and vapor bubble. Most of the sound produced is between 20 and 200 Hz, useful for both seismic and sonar applications, there are also plans to use PSS as a non-lethal weapon against submerged divers. In 1953, the weight dropping Thumper technique was introduced as an alternative to dynamite sources, a thumper truck truck is a vehicle-mounted ground impact system which can be used to provide a seismic source
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Geology
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Geology is an earth science concerned with the solid Earth, the rocks of which it is composed, and the processes by which they change over time. Geology can also refer generally to the study of the features of any terrestrial planet. Geology gives insight into the history of the Earth by providing the evidence for plate tectonics, the evolutionary history of life. Geology also plays a role in engineering and is a major academic discipline. The majority of data comes from research on solid Earth materials. These typically fall into one of two categories, rock and unconsolidated material, the majority of research in geology is associated with the study of rock, as rock provides the primary record of the majority of the geologic history of the Earth. There are three types of rock, igneous, sedimentary, and metamorphic. The rock cycle is an important concept in geology which illustrates the relationships between three types of rock, and magma. When a rock crystallizes from melt, it is an igneous rock, the sedimentary rock can then be subsequently turned into a metamorphic rock due to heat and pressure and is then weathered, eroded, deposited, and lithified, ultimately becoming a sedimentary rock. Sedimentary rock may also be re-eroded and redeposited, and metamorphic rock may also undergo additional metamorphism, all three types of rocks may be re-melted, when this happens, a new magma is formed, from which an igneous rock may once again crystallize. Geologists also study unlithified material which typically comes from more recent deposits and these materials are superficial deposits which lie above the bedrock. Because of this, the study of material is often known as Quaternary geology. This includes the study of sediment and soils, including studies in geomorphology, sedimentology and this theory is supported by several types of observations, including seafloor spreading, and the global distribution of mountain terrain and seismicity. This coupling between rigid plates moving on the surface of the Earth and the mantle is called plate tectonics. The development of plate tectonics provided a basis for many observations of the solid Earth. Long linear regions of geologic features could be explained as plate boundaries, mid-ocean ridges, high regions on the seafloor where hydrothermal vents and volcanoes exist, were explained as divergent boundaries, where two plates move apart. Arcs of volcanoes and earthquakes were explained as convergent boundaries, where one plate subducts under another, transform boundaries, such as the San Andreas Fault system, resulted in widespread powerful earthquakes. Plate tectonics also provided a mechanism for Alfred Wegeners theory of continental drift and they also provided a driving force for crustal deformation, and a new setting for the observations of structural geology
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Paleoseismology
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Paleoseismology looks at geologic sediments and rocks, for signs of ancient earthquakes. It is used to supplement seismic monitoring, for the calculation of seismic hazard, paleoseismology is usually restricted to geologic regimes that have undergone continuous sediment creation for the last few thousand years, such as swamps, lakes, river beds and shorelines. In this typical example, a trench is dug in an active sedimentation regime, evidence of thrust faulting can be seen in the walls of the trench. It becomes a matter of deducting the relative age of each fault, the faults can be dated in absolute terms, if there is dateable carbon, or human artifacts. Many notable discoveries have made using the techniques of paleoseismology. For example, there is a misconception that having many smaller earthquakes can somehow relieve a major fault such as the San Andreas. It is now known that all the movement of the fault takes place with extremely large earthquakes. All of these events, leave some sort of trace in the sedimentation record. Another famous example involves the megathrust earthquakes of the Pacific Northwest and it was thought for some time that there was low seismic hazard in region because relatively few modern earthquakes are being recorded. There was a concept that the zone was merely sliding in a benign manner. All of these notions were shattered by paleoseismology studies showing evidence of extremely large earthquakes. These are caused by the interface between the sea floor stressing the overlaying coastal soils in compression. Paleoseismic investigations are performed through trenching studies in which a trench is dug. Trenching studies are relevant to seismically active regions, such as many parts of California. TERPRO Commission, International Union for Quaternary Research
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Seismogram
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A seismogram is a graph output by a seismograph. It is a record of the motion at a measuring station as a function of time. Seismograms typically record motions in three axes, with the z axis perpendicular to the Earths surface and the x-. The energy measured in a seismogram may result from an earthquake or from other source. Seismograms can record lots of things, and record many little waves and these tiny microseisms can be caused by heavy traffic near the seismograph, waves hitting a beach, the wind, and any number of other ordinary things that cause some shaking of the seismograph. Historically, seismograms were recorded on paper attached to rotating drums, some used pens on ordinary paper, while others used light beams to expose photosensitive paper. Today, practically all seismograms are recorded digitally to make analysis by computer easier, some drum seismometers are still found, especially when used for public display. Seismograms are essential for finding the location and magnitude of earthquakes, prior to the availability of digital processing of seismic data in the late 1970s, the records were done in a few different forms on different types of media. A Helicorder drum is a used to record data into photographic paper or in the form of paper. A piece of paper is wrapped around a drum of the helicorder which receives the seismic signal from a seismometer. For each predefined interval of data, the helicorder will plot the data in one line before moving to the next line at the next interval. The paper must be changed after the helicorder writes on the last line of the paper, in the model that use ink, regular maintenance of the pen must be done for accurate recording. A Develocorder is a machine that record multi-channels seismic data into a 16 mm film, the machine was developed by Teledyne Geotech during the mid 1960s. It can automatically plot seismograms from 18 seismic signal sources and 3 time signals on a reel of film. The signals from seismometers are processed by 15.5 Hz recording galvanometers which record the seismograms to a reel of 200 feet of film at the speeds between 3 and 20 centimetres per minute, the machine has self-contained circulating chemicals that are used to automatically develop the film. However, the machine takes at least 10 minutes from time of recording to the time that the film can be viewed, after the digital processing has been used, the archives of the seismograms were recorded in magnetic tapes. The data from the tapes can then be read back to reconstruct the original waveforms. Unfortunately, due to the deterioration of older magnetic tape medias, today, many other forms are used to digitally record the seismograms into digital medias
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Thales of Miletus
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Thales of Miletus was a pre-Socratic Greek/Phoenician philosopher, mathematician and astronomer from Miletus in Asia Minor. He was one of the Seven Sages of Greece, Thales is recognized for breaking from the use of mythology to explain the world and the universe, and instead explaining natural objects and phenomena by theories and hypothesis, i. e. science. Aristotle reported Thales hypothesis that the principle of nature and the nature of matter was a single material substance. In mathematics, Thales used geometry to calculate the heights of pyramids and he is the first known individual to use deductive reasoning applied to geometry, by deriving four corollaries to Thales theorem. He is the first known individual to whom a mathematical discovery has been attributed, the ancient source, Apollodorus of Athens, writing during the 2nd century BCE, thought Thales was born about the year 625 BCE. The dates of Thales life are not exactly known, but are roughly established by a few events mentioned in the sources. According to Herodotus Thales predicted the eclipse of May 28,585 BC. Diogenes Laërtius quotes the chronicle of Apollodorus of Athens as saying that Thales died at the age of 78 during the 58th Olympiad and attributes his death to heat stroke while watching the games. Plutarch had earlier told this version, Solon visited Thales and asked him why he remained single, nevertheless, several years later, anxious for family, he adopted his nephew Cybisthus. Thales involved himself in many activities, taking the role of an innovator, some say that he left no writings, others say that he wrote On the Solstice and On the Equinox. Diogenes Laërtius quotes two letters from Thales, one to Pherecydes of Syros, offering to review his book on religion, Thales identifies the Milesians as Athenian colonists. He was aware of the existence of the lodestone, and was the first to be connected to knowledge of this in history, according to Aristotle, Thales thought lodestones had souls, because iron is attracted to them. According to Hieronymus, historically quoted by Diogenes Laertius, Thales found the height of pyramids by comparison between the lengths of the shadows cast by a person and by the pyramids, several anecdotes suggest Thales was not just a philosopher, but also a businessman. A story, with different versions, recounts how Thales achieved riches from an olive harvest by prediction of the weather, in one version, he bought all the olive presses in Miletus after predicting the weather and a good harvest for a particular year. Thales’ political life had mainly to do with the involvement of the Ionians in the defense of Anatolia against the power of the Persians. In neighbouring Lydia, a king had come to power, Croesus and he had conquered most of the states of coastal Anatolia, including the cities of the Ionians. The story is told in Herodotus, the war endured for five years, but in the sixth an eclipse of the Sun spontaneously halted a battle in progress. It seems that Thales had predicted this solar eclipse, the Seven Sages were most likely already in existence, as Croesus was also heavily influenced by Solon of Athens, another sage
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Anaximenes of Miletus
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Anaximenes of Miletus was an Ancient Greek Pre-Socratic philosopher active in the latter half of the 6th century BC. One of the three Milesian philosophers, he is identified as a friend or student of Anaximander. Anaximenes, like others in his school of thought, practiced material monism and this tendency to identify one specific underlying reality made up of a material thing is what Anaximenes is principally known for today. The choice of air may seem arbitrary, but Anaximenes based his conclusion on naturally observable phenomena in the processes of rarefaction and condensation, when air condenses it becomes visible, as mist and then rain and other forms of precipitation. As the condensed air cools Anaximenes supposed that it went on to form earth, in contrast, water evaporates into air, which ignites and produces flame when further rarefied. Having concluded that everything in the world is composed of air, Anaximenes used his theory to devise a scheme that explains the origins and nature of the earth and the surrounding celestial bodies. Air felted to create the flat disk of the earth, which he said was table-like and behaved like a leaf floating on air. In keeping with the view of celestial bodies as balls of fire in the sky, Anaximenes proposed that the earth let out an exhalation of air that rarefied, ignited. Similarly, he considered the moon and sun to be flat, in his theory, when the sun sets it does not pass under the earth, but is merely obscured by higher parts of the earth as it circles around and becomes more distant. Anaximenes likens the motion of the sun and the celestial bodies around the earth to the way that a cap may be turned around the head. Anaximenes used his observations and reasoning to provide causes for natural phenomena on the earth as well. In either case the earth becomes weakened by its cracks, so that hills collapse and cause earthquakes, lightning is similarly caused by the violent separation of clouds by the wind, creating a bright, fire-like flash. Rainbows, on the hand, are formed when densely compressed air is touched by the rays of the sun. These examples show how Anaximenes, like the other Milesian philosophers and they sought unifying causes for diversely occurring events, rather than treating each one on a case-by-case basis, or attributing them to gods or to a personified nature. The Anaximenes crater on the Moon is named in his honor, the Earlier Presocratics and the Pythagoreans. The Art and Culture of Early Greece, 1100-480 BC, Early Greek Thought, Before the Dawn. Presocratic Reflexivity, The Construction of Philosophical Discourse, c, the One and Many in Presocratic Philosophy. Washington, DC, Center for Hellenic Studies with Harvard University Press, infinity in the Presocratics, A Bibliographical and Philosophical Study
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Aristotle
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Aristotle was an ancient Greek philosopher and scientist born in the city of Stagira, Chalkidice, on the northern periphery of Classical Greece. His father, Nicomachus, died when Aristotle was a child, at seventeen or eighteen years of age, he joined Platos Academy in Athens and remained there until the age of thirty-seven. Shortly after Plato died, Aristotle left Athens and, at the request of Philip II of Macedon, teaching Alexander the Great gave Aristotle many opportunities and an abundance of supplies. He established a library in the Lyceum which aided in the production of many of his hundreds of books and he believed all peoples concepts and all of their knowledge was ultimately based on perception. Aristotles views on natural sciences represent the groundwork underlying many of his works, Aristotles views on physical science profoundly shaped medieval scholarship. Their influence extended from Late Antiquity and the Early Middle Ages into the Renaissance, some of Aristotles zoological observations, such as on the hectocotyl arm of the octopus, were not confirmed or refuted until the 19th century. His works contain the earliest known study of logic, which was incorporated in the late 19th century into modern formal logic. Aristotle was well known among medieval Muslim intellectuals and revered as The First Teacher and his ethics, though always influential, gained renewed interest with the modern advent of virtue ethics. All aspects of Aristotles philosophy continue to be the object of academic study today. Though Aristotle wrote many elegant treatises and dialogues – Cicero described his style as a river of gold – it is thought that only around a third of his original output has survived. Aristotle, whose means the best purpose, was born in 384 BC in Stagira, Chalcidice. His father Nicomachus was the physician to King Amyntas of Macedon. Aristotle was orphaned at a young age, although there is little information on Aristotles childhood, he probably spent some time within the Macedonian palace, making his first connections with the Macedonian monarchy. At the age of seventeen or eighteen, Aristotle moved to Athens to continue his education at Platos Academy and he remained there for nearly twenty years before leaving Athens in 348/47 BC. Aristotle then accompanied Xenocrates to the court of his friend Hermias of Atarneus in Asia Minor, there, he traveled with Theophrastus to the island of Lesbos, where together they researched the botany and zoology of the island. Aristotle married Pythias, either Hermiass adoptive daughter or niece and she bore him a daughter, whom they also named Pythias. Soon after Hermias death, Aristotle was invited by Philip II of Macedon to become the tutor to his son Alexander in 343 BC, Aristotle was appointed as the head of the royal academy of Macedon. During that time he gave not only to Alexander
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Zhang Heng
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Zhang Heng, formerly romanized as Chang Heng, was a Han Chinese polymath from Nanyang who lived during the Han dynasty. Zhang Heng began his career as a civil servant in Nanyang. Eventually, he became Chief Astronomer, Prefect of the Majors for Official Carriages and his uncompromising stance on historical and calendrical issues led to his becoming a controversial figure, preventing him from rising to the status of Grand Historian. His political rivalry with the palace eunuchs during the reign of Emperor Shun led to his decision to retire from the court to serve as an administrator of Hejian in Hebei. Zhang returned home to Nanyang for a time, before being recalled to serve in the capital once more in 138. He died there a year later, in 139, Zhang applied his extensive knowledge of mechanics and gears in several of his inventions. He improved previous Chinese calculations for pi and his fu and shi poetry were renowned in his time and studied and analyzed by later Chinese writers. Zhang received many honors for his scholarship and ingenuity, some modern scholars have compared his work in astronomy to that of the Greco-Roman Ptolemy. Born in the town of Xie in Nanyang Commandery, Zhang Heng came from a distinguished, at age ten, Zhangs father died, leaving him in the care of his mother and grandmother. An accomplished writer in his youth, Zhang left home in the year 95 to pursue his studies in the capitals of Changan, while traveling to Luoyang, Zhang passed by a hot spring near Mount Li and dedicated one of his earliest fu poems to it. Government authorities offered Zhang appointments to offices, including a position as one of the Imperial Secretaries, yet he acted modestly. At age twenty-three, he returned home with the title Officer of Merit in Nanyang, serving as the master of documents under the administration of Governor Bao De, as he was charged with composing inscriptions and dirges for the governor, he gained experience in writing official documents. As Officer of Merit in the commandery, he was responsible for local appointments to office. He spent much of his time composing rhapsodies on the capital cities, when Bao De was recalled to the capital in 111 to serve as a minister of finance, Zhang continued his literary work at home in Xie. Zhang Heng began his studies in astronomy at the age of thirty and began publishing his works on astronomy, in 112, Zhang was summoned to the court of Emperor An, who had heard of his expertise in mathematics. When he was nominated to serve at the capital, Zhang was escorted by carriage—a symbol of his official status—to Luoyang and he was promoted to Chief Astronomer for the court, serving his first term from 115–120 under Emperor An and his second under the succeeding emperor from 126–132. As Chief Astronomer, Zhang was a subordinate of the Minister of Ceremonies, when the government official Dan Song proposed the Chinese calendar should be reformed in 123 to adopt certain apocryphal teachings, Zhang opposed the idea. He considered the teachings to be of questionable stature and believed they could introduce errors, others shared Zhangs opinion and the calendar was not altered, yet Zhangs proposal that apocryphal writings should be banned was rejected
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Han dynasty
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The Han dynasty was the second imperial dynasty of China, preceded by the Qin dynasty and succeeded by the Three Kingdoms period. Spanning over four centuries, the Han period is considered an age in Chinese history. To this day, Chinas majority ethnic group refers to itself as the Han people and it was founded by the rebel leader Liu Bang, known posthumously as Emperor Gaozu of Han, and briefly interrupted by the Xin dynasty of the former regent Wang Mang. This interregnum separates the Han dynasty into two periods, the Western Han or Former Han and the Eastern Han or Later Han, the emperor was at the pinnacle of Han society. He presided over the Han government but shared power with both the nobility and appointed ministers who came largely from the gentry class. The Han Empire was divided into areas controlled by the central government using an innovation inherited from the Qin known as commanderies. These kingdoms gradually lost all vestiges of their independence, particularly following the Rebellion of the Seven States, from the reign of Emperor Wu onward, the Chinese court officially sponsored Confucianism in education and court politics, synthesized with the cosmology of later scholars such as Dong Zhongshu. This policy endured until the fall of the Qing dynasty in 1911 AD, the Han dynasty was an age of economic prosperity and saw a significant growth of the money economy first established during the Zhou dynasty. The coinage issued by the government mint in 119 BC remained the standard coinage of China until the Tang dynasty. The period saw a number of limited institutional innovations, the Xiongnu, a nomadic steppe confederation, defeated the Han in 200 BC and forced the Han to submit as a de facto inferior partner, but continued their raids on the Han borders. Emperor Wu of Han launched several campaigns against them. The ultimate Han victory in these wars eventually forced the Xiongnu to accept vassal status as Han tributaries, the territories north of Hans borders were quickly overrun by the nomadic Xianbei confederation. Imperial authority was seriously challenged by large Daoist religious societies which instigated the Yellow Turban Rebellion. When Cao Pi, King of Wei, usurped the throne from Emperor Xian, following Liu Bangs victory in the Chu–Han Contention, the resulting Han dynasty was named after the Hanzhong fief. Chinas first imperial dynasty was the Qin dynasty, the Qin unified the Chinese Warring States by conquest, but their empire became unstable after the death of the first emperor Qin Shi Huangdi. Within four years, the authority had collapsed in the face of rebellion. Although Xiang Yu proved to be a commander, Liu Bang defeated him at Battle of Gaixia. Liu Bang assumed the title emperor at the urging of his followers and is known posthumously as Emperor Gaozu, Changan was chosen as the new capital of the reunified empire under Han
